The less trodden path with sqlite

The less trodden path with sqlite

Lately, I've been tinkering around a bit with sqlite3, and this post contains a brief of some exciting set of features and interfaces that I've re-discovered during the process.


sqlite3 isn't exactly new. It's been around for almost two decades now (much longer than I've known coding myself 😛). There's plethora of articles on the internet describing what it is, what makes it special, when you should ideally use it (and when not) and why "it's the only database you will ever need".

So, I'm not gonna repeat all those advices here. Instead I'll try to cover some of other exciting, specialised interfaces that sqlite3 offers and how we can profit from it!


#1 Custom functions in SQL

sqlite3 provides support for registering custom, application-defined functions in C that can be exposed to queries in SQL.

Custom functions can be used to integrate your queries with the rest of the system, or provide access to external functionality. They can be used to perform complex calculation or do statistical operations on data.

Custom functions can be used to marshal / un-marshal and / or manipulate data in alternative formats (such as json). They can be used to compute hashes and do much more!

sqlite3 comes with a bunch of built-in scalar and aggregate functions, and with routines to manipulate date / time information. There is also nalgeon/sqlean on Github that aims to pack a lot more set of functions into convenient extensions that can be loaded on-demand.

#2 Virtual Tables

Virtual Tables are one of the most exciting features of sqlite3 that I found! Essentially, virtual table allows us to register objects with sqlite3 that, from the perspective of an SQL statement, looks like any other table / view.

This allows the user to be able to query them just as they would do with any other table / view. When a user executes a query against a virtual table sqlite3 invokes callback methods on the virtual table object.

This has the potential to unlock a ton of different use-cases. For eg., there's an official csv virtual table implementation that allow users to query data from a csv file just as they would with a normal table. There are other implementations that make it possible to interface with other systems, such as augmentable-dev/askgit uses sqlite3 virtual tables to allow users to query data from git repositories, osquery.io that allow users to run queries against the operating systems!

#3 Authorizer to check / limit what a user can query

sqlite3 supports registering a custom authorizer function that can limit what data a query can access and what action can it perform. This, together with other defensive settings, can be used to turn sqlite3 into a sandbox where we can execute raw, user-supplied, untrusted statements with confidence that it won't cause any un-intended side-effects.

#4 Capture a change-set of differences in a session

sqlite3 session extension provides a mechanism for recording changes to some or all of the rowid tables in an SQLite database, and packaging those changes into a "changeset" file that can later be used to apply the same set of changes to another database. This can be used to capture diffs generated by multiple users, working asynchronously, capturing and merging it together into a unified database file.

Sessions can be useful when sqlite3 is used as an application data format and multiple users work on the same file at the same time. Each user can generate a patch for their side of work (much like how one would do it with a vcs like git) and the system could combine those into a unified database file.

#5 Online Backup API

sqlite3Online Backup API allows the contents of one database to be copied into another database, in an incremental fashion where the database is only locked for the brief periods of time when it is actually being read from. This allows other database users to continue uninterrupted while a backup of an online database is made.

It provides a basic building block onto which other, more complex replication solutions can be built.

#6 Dynamically loaded extensions

sqlite3 has support for building extensions that can either be loaded dynamically (as a shared object file) or linked statically into the final build.

The sqlite3ext.h defines the Extension API which the module can use to interact with the database connection (that loaded the extension). The extension can do pretty much anything that you could normally do with the sqlite3 core, like registering custom functions, defining virtual table modules and more!

One should consider building re-usable modules as loadable extensions to allow for easier distribution and better re-use.


In the follow-up articles, I would try to cover some of the features I've discussed here and how one can leverage those while coding with Golang using http://go.riyazali.net/sqlite, a library that I recently open-sourced 🤗

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